internal/profile: optimize Parse allocs

In our case, it greatly improves the performance of continuously collecting diff profiles from the net/http/pprof endpoint, such as /debug/pprof/allocs?seconds=30.

This CL is a cherry-pick of my PR upstream: google/pprof#951

Benchmark of profile Parse func:

goos: linux
goarch: amd64
pkg: github.com/google/pprof/profile
cpu: 13th Gen Intel(R) Core(TM) i7-1360P
         │ old-parse.txt │            new-parse.txt             │
         │    sec/op     │    sec/op     vs base                │
Parse-16    62.07m ± 13%   55.54m ± 13%  -10.52% (p=0.035 n=10)

         │ old-parse.txt │            new-parse.txt             │
         │     B/op      │     B/op      vs base                │
Parse-16    47.56Mi ± 0%   41.09Mi ± 0%  -13.59% (p=0.000 n=10)

         │ old-parse.txt │            new-parse.txt            │
         │   allocs/op   │  allocs/op   vs base                │
Parse-16     272.9k ± 0%   175.8k ± 0%  -35.58% (p=0.000 n=10)

Change-Id: I737ff9b9f815fdc56bc3b5743403717c4b6f07fd
GitHub-Last-Rev: a09108f
GitHub-Pull-Request: #76145
Reviewed-on: https://go-review.googlesource.com/c/go/+/717081
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>

Author: pymq

src/internal/profile/proto.go

@@ -24,6 +24,7 @@ package profile
 import (
 	"errors"
 	"fmt"
+	"slices"
 )
 
 type buffer struct {
@@ -175,6 +176,16 @@ func le32(p []byte) uint32 {
 	return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
 }
 
+func peekNumVarints(data []byte) (numVarints int) {
+	for ; len(data) > 0; numVarints++ {
+		var err error
+		if _, data, err = decodeVarint(data); err != nil {
+			break
+		}
+	}
+	return numVarints
+}
+
 func decodeVarint(data []byte) (uint64, []byte, error) {
 	var i int
 	var u uint64
@@ -275,6 +286,9 @@ func decodeInt64(b *buffer, x *int64) error {
 func decodeInt64s(b *buffer, x *[]int64) error {
 	if b.typ == 2 {
 		// Packed encoding
+		dataLen := peekNumVarints(b.data)
+		*x = slices.Grow(*x, dataLen)
+
 		data := b.data
 		for len(data) > 0 {
 			var u uint64
@@ -305,8 +319,11 @@ func decodeUint64(b *buffer, x *uint64) error {
 
 func decodeUint64s(b *buffer, x *[]uint64) error {
 	if b.typ == 2 {
-		data := b.data
 		// Packed encoding
+		dataLen := peekNumVarints(b.data)
+		*x = slices.Grow(*x, dataLen)
+
+		data := b.data
 		for len(data) > 0 {
 			var u uint64
 			var err error